Asynchronous image downloads

There are a few finicky details that need to be paid attention to when lazily loading images in the background, like:

not starting too many parallel downloads

handling the concurrency

cancelling downloads when images are not needed anymore

I recommend to use one of the two popular libraries that do the trick in just a few lines of code: SDWebImage and Kingfisher. Both are easy to get via Cocoapods and are quite similar in API and supported features.

SDWebImage

After adding the SDWebImage pod to the project:

pod 'SDWebImage'

Given an image view and a URL to load the image from, loading the image with an activity indicator is as simple as:

There is a nice cheat sheet that demonstrates common cases like customizing the indicator or getting notified when downloads complete.

Differences between the libraries: SDWebImage vs. Kingfisher

SDWebImage supports progressive display of the image while loading is in progress via options: [.progressiveDownload] which is nice when loading larger images. This is not supported in Kingfisher as of now.

SDWebImage has support for Webp images included as a subspec that works out of the box, while for Kingfisher it’s an external plugin KingfisherWebP that requires extra configuration - for applications that have a lot of resources, webp helps delivering higher quality with shorter download times.

SDWebImage is implemented in Objective-C, Kingfisher in Swift 4.

Example project

Here is an example project that includes a UITableViewController example for both libraries which are included via Cocoapods: